The present invention relates to a method of determining the distance of a projection point of a first imaging beam of an imaging device on a surface of a printing form from a measuring point of a measuring device on the surface of the printing form, both the projection point of the first imaging beam and the measuring point of the measuring device being movable in relation to the surface of the printing form, and the position of the imaging beam and the position of the measuring point with respect to the surface of the printing form in relation to a fixed point being determinable. Furthermore, the present invention relates to a method of determining the distance of a first projection point of a first imaging beam of a first imaging device on a surface of a printing form from a second projection point of a second imaging beam of a second imaging device on the surface of the printing form.
For creating an image on a printing form, whether it is accomplished in a printing form imager or a direct imaging printing unit, using an imaging beam, in particular a laser light beam, the distance between the imaging beam source of the imaging device and the surface of the printing form must be known with sufficient precision so that appropriate imaging parameters of the imaging beam may be set to produce a printing dot of desired size at the location of the projection point of the imaging beam. One important method is to determine the distance between the imaging device and the surface of the printing form directly using a measuring device. A measured value obtained in this way may be processed in a control unit of the imaging device to change the optical path from the imaging device to the surface, in an auto-focus system for an imaging optic, for example, or to select specific parameters of the light source, such as the necessary intensity of the imaging light. The measuring point of the measuring device is generally not automatically at the position of the projection point of the imaging beam at the time of the measurement, rather the position of the measuring point is reached by the projection point only at a later time, after (typically uniform) movement of the imaging device in relation to the surface of the printing form. For the triggering of the imaging beam to actually occur precisely at the time at which the projection point reaches the position of the measuring point at the instant of the measurement, precise knowledge of the distance of the projection point from the measuring point at the instant of the measurement is necessary if the (uniform) relative speed is known. The imaging result reacts particularly sensitively to deviations from the precise triggering instant in regions of the surface of the printing form on a rotating body, in particular on a printing form cylinder, in which the printing form does not rest uniformly on the outer surface of the rotating body (e.g., plate bubble).
In order to reduce the time for producing a complete image on the surface of a printing form, often multiple imaging beams are used for simultaneously writing printing dots in parallel. The imaging beams may be positioned individually or in groups on one or more imaging devices. In order for a printing image to be produced by multiple imaging beams whose printing dots have the same position in relation to one another as if they had been produced by only one imaging beam, the distances of the projection points of the imaging beams at a point in time must be known precisely, so that a changed position of the printing dots is possibly achieved through delayed or advanced triggering of the imaging beam at known (uniform) speed. In this way, shifts and/or gaps or overlaps or the like which would occur in the printing image if the triggering was unchanged may be compensated for.
An obvious possibility for managing or even avoiding the problem described above is to determine the distance of the projection points of the imaging beam or the distances of the projection points of the imaging beams from the measuring point of the measuring device in a calibration measurement during the assembly or manufacture of the imaging device and the distance measuring device, which is preferably integrated into the imaging device. Calibration may also be performed during assembly of multiple imaging devices, frequently having a shared linear actuator system, to form an imaging unit. Precision measurement having subsequent electronic (triggering instant) or mechanical (position variation) correction may be performed using an optical bench and beam measuring devices. It is disadvantageous that this method is relatively complicated and generally may no longer be used for adjustment of the imaging devices when they are incorporated into the printing unit, in particular in the field at the client, a situation in which conditions different from those on an optical bench exist. In addition, a single setting before delivery may not take into consideration possible influences of aging, temperature, or the like, which only occur during operation.
An array of solutions is already known which allow potential use at the client. For example, in German Patent Application No. 102 03 694.2, a method of determining the relative position of a first and a second imaging device to one another is described, in which surface coverages of a series of combination patterns imaged by both imaging devices are compared to surface coverages of reference patterns imaged by one imaging device. The comparison may advantageously be performed on a printed printing material. An identified combination pattern having identical surface coverage to the reference pattern is uniquely assigned a deviation from a setpoint distance.
It is known, from German Patent Application No. 44 37 284 A1, for example, that a calibration of a controller for the deflection of a laser beam may be performed as follows. A light-sensitive medium is irradiated by the laser beam to produce a test image and digitized image sections are subsequently produced from this test image, which are recorded by a CNC-controlled camera. Correction data for controlling the deflection of the laser beam is computed on the basis of a comparison of the actual positions of the laser beam measured by the recording of the image sections with predetermined setpoint positions. The use of this method has the disadvantage, among other things, of requiring the use of precise CNC control for the camera, which is therefore costly.
A device and a method for calibrating beam scanning devices is disclosed in German Patent Application No. 197 32 668 A1, in which a surface having defined markings is scanned in such a way that a detector device may produce a signal when a marking is detected by the scanned beam. A correction signal for the beam deflection may be produced using a comparison between the detector signals and the corresponding setpoint positions.
A device and a method for automatic alignment of an inkjet printer cartridge also exists in this context. For example, European Patent Application No. 0 540 244 A2 describes that an optical sensor having a quad photodiode may be mounted on the carriage of an inkjet printer. Using the optical sensor, the position of horizontal and vertical test lines, which are printed using the inkjet in a test procedure, may be determined, so that a correction to a setpoint position may be performed, in particular mechanically as a position correction of the cartridge.
The technical teachings of the procedures described are not directly applicable to devices for producing images on printing forms in printing units.